RealityRipple Software has many extensions for Mozilla products and derivatives available. Some of them have been fairly popular over the years. Hopefully you'll find them useful.
Notice: These extensions are now "out of date" and may not work correctly on the latest versions of Firefox. At this time, I have no interest in converting them to the new, "Chrome-clone" API, and thus they are defunct. Most of them have now been ported to Pale Moon.